1. Field of the Invention
The present invention relates generally to an optical information storage device, and more particularly to an optical information recording and reproducing device for recording optical signals on both lands and grooves formed on a recording medium and reproducing the recorded optical signals from the recording medium.
2. Description of the Related Art
An optical disk has received attention as a memory medium that becomes a core in the recent rapid development of multimedia, and it is usually accommodated in a cartridge case to be provided as an optical disk cartridge for practical use. The optical disk cartridge is loaded into an optical disk drive to perform reading/writing of data (information) from/into the optical disk by means of an optical pickup (optical head).
A recent optical disk drive intended to realize size reduction is composed of a fixed optical assembly including a laser diode module, a polarization beam splitter for reflecting and transmitting a laser beam, and a photodetector for receiving reflected light from an optical disk, and a movable optical assembly including a carriage and an optical head having an objective lens and a beam raising mirror mounted on the carriage. The carriage is movable in the radial direction of the optical disk along a pair of rails by means of a voice coil motor.
A write-power laser beam emitted from the laser diode module of the fixed optical assembly is first collimated by a collimator lens, next transmitted by the polarization beam splitter, next reflected by the beam raising mirror of the optical head, and finally focused on the optical disk by the objective lens, thereby writing data onto the optical disk. On the other hand, data reading is performed by directing a read-power laser beam onto the optical disk. Reflected light from the optical disk is first collimated by the objective lens, next reflected by the polarization beam splitter, and finally detected by the photodetector, thereby converting the detected optical signal into an electrical signal.
A plurality of grooves are formed on a substrate of the optical disk in a concentric or spiral fashion to guide a laser beam to be directed onto the substrate. A flat portion defined between any adjacent ones of the grooves is called a land. In a general optical disk in the prior art, either the lands or the grooves are used as recording tracks on which information is recorded. However, a recent important technical subject to be considered is to increase a recording density by using both the lands and the grooves as the recording tracks to thereby decrease a track pitch. In this respect, various methods for realizing this subject have already been proposed.
In a magneto-optical disk drive as a kind of optical disk drive, a magneto-optical signal recorded on a magneto-optical disk is reproduced by directing a read-power laser beam onto the magneto-optical disk and differentially detecting a P-polarized light component and an S-polarized light component of reflected light from the magneto-optical disk by a method well known in the art. In this manner, the magneto-optical signal must be optimally reproduced by differentially detecting the P-polarized light component and the S-polarized light component of the reflected light. However, individual magneto-optical disk drives have differences in characteristics of their optical components, causing a phase difference between the P-polarized light component and the S-polarized light component of the reflected light in each magneto-optical disk drive. Further, a difference in kind between recording media also causes a similar phase difference.
FIG. 1 is a graph showing the relation between phase difference and carrier-to-noise ratio (CNR) in a 640-MB (megabytes) magneto-optical disk and in a 1.3-GB (gigabytes) magneto-optical disk. As apparent from FIG. 1, a phase difference giving a maximum value of the CNR is present in each of the 640-MB magneto-optical disk and the 1.3-GB magneto-optical disk. While the graph of FIG. 1 further shows that the CNR in the 1.3-GB magneto-optical disk higher in recording density is more insensitive to the phase difference, the 1.3-GB magneto-optical disk has a problem that the magneto-optical signal (MO signal) is largely undulated.
FIG. 2 is a graph showing the relation between phase difference and MO undulation/MO amplitude in a 640-MB magneto-optical disk and in a 1.3-GB magneto-optical disk. The MO undulation means that the envelope of an MO signal in one revolution of the disk is undulated. Such MO undulation is shown in FIG. 3. The MO undulation causes a deterioration in jitter in cutting an MO signal at a certain slice level. As apparent from FIG. 2, the MO undulation in the 1.3-GB magneto-optical disk steeply changes with a change in phase difference. Accordingly, the phase difference must be adjusted to obtain an optimum reproduced signal quality.
Further, in a magneto-optical disk drive for recording information on both the lands and the grooves of a recording medium, the width of each track is smaller than the diameter of a beam spot to be formed on the recording medium, so that the track covered by the beam spot is largely influenced by crosstalk from the adjacent track. Thus, such a land/groove recording method has a problem such that an undesirable light component reflected from any adjacent groove or land is increased to cause an associated phase difference, and a resultant change in polarization state of reproduced light. As a result, information cannot be well reproduced from the magneto-optical recording medium.
FIG. 4 is a graph showing the relation between phase difference and CNR in performing land reproduction and groove reproduction. As apparent from FIG. 4, the CNR changes with a change in phase difference in each of land reading and groove reading, and an optimum phase difference giving a maximum CNR is present in each case. Accordingly, it is necessary to perform phase compensation of polarized light components of reproduced light in each of land reading and groove reading, thereby obtaining an optimum phase difference between the P-polarized light component and the S-polarized light component. For example, Japanese Patent Laid-open Nos. 9-282730, 9-282733, and 10-134444 disclose techniques for switching a phase difference between polarized light components of reproduced light between in land reproduction and in groove reproduction. However, each technique employs a complex optical system.
It is therefore an object of the present invention to provide an optical information storage device which can reproduce information well by providing different phase compensation amounts for land reading and groove reading with a relatively simple and inexpensive optical system.
In accordance with an aspect of the present invention, there is provided an optical information storage device for directing an incident light beam onto a recording medium having a recording surface composed of lands and grooves as tracks and detecting a reproduced signal from a reflected light beam from said recording medium, comprising a phase plate provided in an optical path of said reflected light beam so as to be tiltable between a first position where said phase plate gives to said reflected light beam a first phase compensation amount required for detection of signals from said lands and a second position where said phase plate gives to said reflected light beam a second phase compensation amount required for detection of signals from said grooves; and drive means for tilting said phase plate.
Preferably, the optical information storage device further comprises control means for supplying a first control signal to said drive means to tilt said phase plate to said first position when said lands are selected as said tracks, and for supplying a second control signal to said drive means to tilt said phase plate to said second position when said grooves are selected as said tracks.
The phase plate is fixed to a housing. The housing is pivotably moved by the drive means. Examples of the drive means include a solenoid, reversible DC motor, and voice coil motor. A first stopper and a second stopper are provided to stop the phase plate at the first position and the second position, respectively. Preferably, the first stopper and the second stopper are adjustable.
In the case of adopting a reversible DC motor as the drive means, a third position between the first position and the second position can be detected by the combination of a magnet and a Hall element. In the case of adopting a voice coil motor as the drive means, the phase plate can be stopped at an arbitrary position between the first position and the second position, so that a desired phase difference can be easily realized. Accordingly, even in the case that the adjustment is not carried out in assembling the device, an arbitrary phase difference can be realized after assembling the device.
In accordance with another aspect of the present invention, there is provided an optical information storage device comprising a housing having a base; an optical recording medium rotatably accommodated in said housing and having a recording surface composed of lands and grooves as tracks; a light source mounted on said base; an optical head having an objective lens for focusing an incident light beam emitted from said light source onto said recording surface of said optical recording medium; a photodetector mounted on said base for detecting a reproduced signal from a reflected light beam from said optical recording medium; a phase plate provided in an optical path of said reflected light beam so as to be tiltable between a first position where said phase plate gives to said reflected light beam a first phase compensation amount required for detection of signals from said lands and a second position where said phase plate gives to said reflected light beam a second phase compensation amount required for detection of signals from said grooves; and drive means for tilting said phase plate.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.